This invention relates to electrical contact devices, and more particularly to a contact spring adapted for sliding electrical contact with the resistance and collector strips of a variable resistance device.
The contact strip embodied by the present invention is intended for use in a variable resistance device of the type having a flat resistance strip connected at one end to a terminal, and a collector strip running parallel to one or both sides of the resistance strip and connected to a second terminal. The contact spring effects a local short circuit between the resistance and collector strips, the total resistance between the two terminals being determined primarily by the location of the spring. Means are provided to move the spring across the surfaces of the strips, thereby enabling the resistance between the two terminals to be varied from a very low value to one that reflects substantially the full value of the resistance strip.
It has been found that the longevity of the spring depends to a considerable extent upon the contact force between the spring and the resistance and collector strips. Should the contact not be firm enough, the passage of large amounts of current between the spring and strips can produce sufficient heating to cause surface deterioration, thereby restricting the amount of effective surface area available for current transfer and further aggravating the heating problem. Continued operation at high currents can lead to unacceptable variances in the output resistance and eventual complete failure of the device. While numerous ways are available to increase the spring contact force, such as the use of leaf spring arrangements, the space available for the spring may be so small as to place severe restrictions on the permissible size of the contact spring and preclude the use of more elaborate devices.
One type of spring that has been found to successfully overcome the space limitation problem consists of a simple base member with a cantilever spring arm depending downward at an acute angle to the base member to contact the resistance strip, and additional cantilever spring arms for contacting the collector strips depending from the base member in crisscross relation to the first mentioned spring arm and on either side thereof. While this arrangement effectively utilizes the small space available, the spring forces developed may not be sufficient to prevent deterioration at the points of contact between the spring arms and the resistance and contact strips when large currents are carried by the resistance device.